Why does Mary Grace attack Mrs. Turpin in "Revelation"?

It appears that people in the doctor's waiting room were sitting quietly until Mrs. Turpin comes in with her husband Claud and begins to talking to others in the room. We learn that Mary Grace is not happy with the chit chat:

She appeared annoyed that anyone should speak while she tried to read.

Further, when the woman that the overweight Mrs. Turpin has cruelly labelled "white trash" mutters under her breath about hogs, she could be describing Mrs. Turpin's behavior in the waiting room:

"A-gruntin and a-rootin and a-groanin."

Both in her speech and her private reflections, Mrs. Turpin passes smug judgment on everyone, including Mary Grace:

The poor girl's face was blue with acne and Mrs. Turpin thought how pitiful it was to have a face like that at that age.

Mrs. Turpin then goes on to reflect on how much better her face is than Mary Grace's.

We hear the story entirely from the point-of-view of Mrs. Turpin, who is supremely complacent and self-satisfied with her own perfections and believes she is superior to the others there. She sees Mary Grace scowling at her in a way that is cuing her to shut up, but Mrs. Turpin is so sure of her own perfections that she blames Mary Grace for being rude. She can't see that she is the obnoxious and annoying one. Finally, when Mrs. Turpin has the following amazing (and comic) outburst in the middle of the waiting room, that goes from thought to speech, Mary Grace has had it:

"'Thank you, Jesus, for making everything the way it is!' It could have been different!" [these are Mrs. Turpin's thoughts] For one thing, somebody else could have got Claud. At the thought of this, she was flooded with gratitude and a terrible pang of joy ran through her. "Oh thank you, Jesus, Jesus, thank you!" she cried aloud.

It is at this point that that Mary Grace attacks her.

O'Connor is manipulating point-of-view. We are receiving everything from Mrs. Turpin's limited perspective, so the attack simply seems a result of Mary Grace being crazy and unpleasant. Mrs. Turpin has so little self awareness she doesn't realize how inane, stupid, and racist she sounds. Rather than take the odd looks she gets from Mary Grace and the woman she labels "white trash" as indications she is being a jerk, she can't imagine this could be so.

We can understand Mary Grace (an agent of God's grace) finally attacking in rage when Mrs. Turpin thanks God that this world is the way it is when it is so obviously a fallen place full of sickness (after all they are all in a doctor's office!), injustice, and pain. Mrs. Turpin is deeply in need of spiritual healing, but she is oblivious to her own disease. At this point she is no better than one of her hogs.

From the opening of her short story “Revelation,” Flannery O’Connor makes it clear that her main character, Mrs. Turpin, is an imposing figure, both physically and mentally. As the story begins, Mrs. Turpin enters her doctor’s office, a small room that, O’Connor’s narrator suggests, is quickly filled by the patient’s large frame: “She stood looming at the head of the magazine table. . . a living demonstration that the room was inadequate and ridiculous.”

With this opening, O’Connor, a particularly cynical author, lets the reader know that Mrs. Turpin is a figure to be taken seriously. This initial description is immediately reinforced by the author’s reference to Mrs. Turpin’s husband, Claud, who is depicted as a subordinate figure in the relationship and the one in need of medical attention. In addition, Mrs. Turpin is racist, evident by her suggestion, made to herself, fortunately, that the “white trash” she had encountered was “worse than niggers.”

Mrs. Turpin is the quintessential Flannery O’Connor character: religious, condescending, ignorant, self-righteous and hypocritical. Mary Grace, in contrast, is well-educated—she attends Wellesley College, her mother informs Mrs. Turpin—but she exudes the kind of anger and bitterness occasionally associated with college students whose exposure to a little knowledge leads to an exaggerated sense of self-importance.

One can only speculate for the reason Mary Grace attacked Mrs. Turpin. Mary Grace could not read Mrs. Turpin’s mind, so she was not privy to the latter’s racist and judgmental thoughts. What Mary Grace did know was that Mrs. Turpin espoused religious views that may have run counter to the college student’s more agnostic or atheistic perspective. Just as Mrs. Turpin condemned what she viewed as “white trash,” Mary Grace may very well have viewed Mrs. Turpin in that same pejorative light. Negative perceptions of another individual aside, there is no valid reason for Mary Grace to have physically assaulted Mrs. Turpin (although, in today’s society, assaulting someone for their beliefs is becoming mainstream). The attack was probably the result of pent-up frustrations over what Mary Grace perceived as the ramblings of an intellectually and culturally inferior person masked behind overt religiosity.

Why does Mary Grace choose Mrs. Turpin for her message in "Revelation"?

Short Answer: Mary Grace's message to "Go back to hell where you came from, you old wart hog" strikes the "target" of Mrs. Turpin because this woman is such a self-righteous hypocrite. 

Mary Grace, whose name signifies her role in the narrative, has listened long enough to Mrs. Turpin to realize that although this woman has been supposedly "saved," she has an uncharitable perspective and feels herself the superior of blacks and poorer whites and has been speaking to Mary Grace's mother and others in the waiting room of a doctor in her self-righteous tones. So in disgust, Mary Grace hurls the book she has been reading at Mrs. Turpin, then falls to the floor, succumbing to a seizure.

The girl's eyes seemed lit all of a sudden with a peculiar light, an unnatural light like night road signs give. 

Mrs. Turpin, who feels that the "girl did know her" because of the light in her eyes, bends down and asks her "What you got to say to me?" It is then that Mary Grace provides Mrs. Turpin the "revelation" that she anticipates, only what Mrs. Turpin hears is not what she has expected. Nevertheless, she has been "singled out for the message" and she ponders it until this revelation alters her perceptions as "a visionary light settled in her eyes." Mrs. Turpin envisions herself and her husband and people like her bringing up the rear in the procession to Heaven.